1. Field of the Invention
The present invention relates generally to methods and devices for measuring oxygen saturation of a patient's blood, and more particularly to methods and devices for performing pulse oximetry.
2. Related Art
Plethysmography is a generic term referring to a variety of techniques for monitoring volume changes, for example, volume changes of the lungs due to respiration, or of blood vessels of a limb or tissue segment. When applied to measurements of blood volume, changes occur in a pulsatile manner with each beat of the heart as blood flows in and out of a portion of the body. The study of vascular activity by fluid displacement methods dates back to at least 1890. More contemporary techniques include strain gauge, pneumatic, impedance, doppler, and photoelectric plethysmography. A plethysmography device produces a waveform that is similar to an arterial pressure waveform. The waveform is useful in measuring pulse velocity and indicating arterial obstructions.
FIG. 1 illustrates an exemplary plethysmograph 100, which includes a waveform 102 produced by a plethysmography device. For timing reference, an electrocardiogram (ECG) signal 104 is illustrated. Waveform 102 provides a measure of the volume of the arterial vasculature. A measure of arterial pulse amplitude is derived from it. A few tens to a few hundreds of milliseconds after the QRS complex, the plethysmography voltage reaches a minimum and starts to increase. This is due to the increasing blood volume in the arterioles as the systolic pulse reaches the periphery. The delay is influenced by the distance that the sensor is placed from the heart. It requires approximately 100 msec for the waveform to reach its maximum. The excursion from minimum to maximum represents the arterial pulse amplitude. During diastole, the recoil of the elastic arterial vessels continues to force blood through the capillaries, so that blood flows through the capillary bed throughout the entire cardiac cycle.
A photoplethysmography device (PPG) (also called a pseudoplethysmography or photoelectric plethysmography device) includes a light detector and a light source. The PPG utilizes the transmission or reflection of light to demonstrate the changes in blood perfusion. Such devices might be used in the cardiology department or intensive care department of a hospital or in a clinic for diagnostic purposes related to vascular surgery. A photoplethysmography device is also referred to, herein, simply as a plethysmography device.
An exemplary circuit 200A for a conventional photoplethysmography device is shown in FIG. 2A. An exemplary mechanical arrangement 200B for a conventional photoplethysmography device is shown in FIG. 2B. In these examples, the light source includes a light-emitting diode (LED) 202, although in alternative models an incandescent lamp can be used. The light detector in this example includes a photoresistor 204 excited by a constant current source. Changes in light intensity cause proportional changes in the resistance of photoresistor 204. Since the current through photoresistor 204 is constant in this example, the resistance changes produce varying analog voltage (Vout—analog) at the output terminal. This varying analog voltage (Vout—analog) is typically converted to a digital signal (Vout—digital) using an analog to digital converter (A/D) 206. Other known light detectors include photo diodes, photo transistors, photo darlingtons and avalanche photo diodes.
Light may be transmitted through a capillary bed such as in an ear lobe or finger tip. As arterial pulsations fill the capillary bed the changes in volume of the blood vessels modify the absorption, reflection and scattering of the light. Stated another way, an arterial pulse in, for example, a finger tip, or ear lobe, causes blood volume to change, thereby changing the optical density of the tissue. Therefore, the arterial pulse modulates the intensity of the light passing through the tissue. Light from LED 202 is reflected into photoresistor 204 by scattering and/or by direct reflection from an underlying bone structure. Such a PPG does not indicate “calibratable” value changes. Thus, its usefulness is generally limited to pulse-velocity measurements, determination of heart rate, and an indication of the existence of a pulse (e.g., in a finger). Additionally, a conventional PPG provides a poor measure of changes in volume and is very sensitive to motion artifacts.
It is noted that photoplethysmography devices may operate in either a transmission configuration or a reflection configuration. In the transmission configuration, LED 202 and the photodetector 204 face one another and a segment of the body (e.g., a finger or earlobe) is interposed between them. In the reflection configuration, LED 202 and photodetector 204 are mounted adjacent to one another, e.g., on the surface of the body, as shown in FIG. 2B.
Pulse oximetry combines the principles of optical plethysmography and spectrophotometry to determine arterial oxygen saturation values. Optical plethysmography, as just explained above, uses light absorbance technology to reproduce waveforms produced by pulsating blood. Spectrophotometry uses various wavelengths of light to perform quantitative measurements about light absorption through given substances. Using these two principles, the arterial oxygen saturation of a patient's blood can be estimated. Arterial oxygen saturation measurements can be used, for example, to monitor and assess heart failure, sleep apnea, and pulmonary function.
Conventional two wavelength pulse oximeters (which perform pulse oximetry) emit light from two LEDs into a pulsatile tissue bed and collect the transmitted light with a photodetector positioned on an opposite surface (transmission pulse oximetry), or an adjacent surface (reflectance pulse oximetry). The “pulse” in pulse oximetry comes from the time varying amount of arterial blood in the tissue during the cardiac cycle, and the processed signals from the photodetector create the familiar plethysmographic waveform due to the cycling light attenuation. For estimating oxygen saturation, at least one of the two LEDs' primary wavelength is chosen at some point in the electromagnetic spectrum where the absorption of oxyhemoglobin (HbO2) differs from the absorption of reduced hemoglobin (Hb). The second of the two LEDs' wavelength must be at a different point in the spectrum where, additionally, the absorption differences between Hb and HbO2 are different from those at the first wavelength. Commercial pulse oximeters typically utilize one wavelength in the red part of the visible spectrum near 660 nanometers (nm), and one in the near infrared part of the spectrum in the range of 880 nm-940 nm. Photocurrents generated within the photodetector are detected and processed for measuring the modulation ratio of the red to infrared signals. This modulation ratio has been observed to correlate well to arterial oxygen saturation. Pulse oximeters are empirically calibrated by measuring the modulation ratio over a range of in vivo measured arterial oxygen saturations (SaO2) on a set of patients, healthy volunteers, or animals. The observed correlation is used in an inverse manner to estimate saturation (SpO2) based on the real-time measured value of modulation ratios. (As used herein, SaO2 refers to the in vivo measured functional saturation, while SpO2 is the estimated functional saturation using pulse oximetry.)
A pulse oximeter resembles the plethysmography device shown in FIG. 2B, except two LEDs (e.g., a red (660 nm) LED and an infrared (940 nm) LED) are used to transmit light through a vascular bed to the photodetector. The difference in the intensity of transmitted light between red and infrared light is caused by the differences in the absorption of light by oxygenated (saturated) and deoxygenated (desaturated) hemoglobin. The resulting voltage difference is used to estimate the amount of oxygen saturation by, for example, comparing the voltage difference to a table.
A problem with current methods and devices for performing pulse oximetry is that they are acutely sensitive to sensor and/or tissue motion. Even rather subtle motion can (and often do) swamp the detected optical signals and render the oxygen saturation measurements unusable. For example, with pulse oximetry devices that measure blood volume changes in a fingertip or earlobe, such subtle motion may be caused by a patient's slight movement of their finger or head. Accordingly, there is a need to reduce motion-induced noise in pulse oximetry.